Immune checkpoint blockade therapy has delivered unprecedented success in the treatment of melanoma and lung cancer. However, as exciting as this is, even with combined inhibition of PD-1 and CTLA4, only a portion of cancer patients were observed with objective responses and an even smaller percentage of them reached long term remission. Therefore, it is imperative to identify specific mechanisms that determine the efficacy of checkpoint targeting, and to develop novel therapeutic strategies synergizing with current treatments. Although the mechanism of checkpoint blockade resistance has not been precisely determined by changes in any novel biomarkers, a consensus has been reached that more favorable responses are observed in patients with T cell-inflamed ?hot? tumors. At the molecular level, the inflamed tumor microenvironment is characterized by activation of T and NK cells, effector molecules for cytolytic functions, chemokines for T and NK cell recruitment, type I interferons (IFN-I), and interferon-responsive genes. The anti-virus role of IFN-I was discovered decades ago, whereas its anti-tumor mechanism was more recently elucidated. The emerging scientific premise supports the hypothesis that there exists a plausible strategy to improve the immune ?readiness? of a tumor, and to overcome tumor resistance to checkpoint blockade therapy by elevating the level of intratumoral IFN-I. In this regard, our preliminary results show that inhibiting the expression of one epigenetic modifier, ubiquitin like with PHD and ring finger domains 1 (UHRF1), in lung cancer cells dramatically triggers IFN-I responses and ultimately intratumoral T cell accumulation. Surprisingly, UHRF1 deficient tumor cells also become resistant to IFN-I-induced PD-L1 surface expression. Furthermore, genetic deletion of UHRF1 impairs the proliferation and function of regulatory T cells (iTregs), a stromal cell population in the tumor mass that carries out an immunosuppressive function. Taken together, we hypothesize that targeting UHRF1 represents a comprehensive strategy to reverse immunosuppression in the tumor microenvironment. In this study, we will test this hypothesis by three specific aims. Aim 1 will determine molecular mechanisms through which tumoral UHRF1 remodels the tumor microenvironment. Aim 2 will determine the mechanism by which UHRF1 regulates T cell activation. Aim 3 will determine the pre-clinical efficacy of a therapeutic strategy combining UHRF1 suppression and PD-1 or CTLA-4 blockade against lung cancer. Since a small molecule UHRF1 inhibitor prototype has been developed, the success of this project will establish a novel and feasible target for tumor microenvironmental reprogramming, and lay a scientific foundation for combination therapy with checkpoint blockade against lung cancer.